1. Field of the Invention
This invention relates to tracking control of transducers which scan successive tracks to reproduce previously recorded information and, more particularly, to tracking control apparatus used in an automatic track following (ATF) system which relies upon previously recorded pilot signals to produce a tracking error indication.
2. Description of the Prior Art
U.S. Pat. No. 4,509,083, assigned to the assignee of the instant invention, describes a prior art proposal of an ATF system employing pilot signals of different frequencies for the purpose of achieving tracking control of transducers which scan previously recorded record tracks on a record medium. Although the tracking control technique described therein exhibits ready application for numerous types of information storage systems, it is particularly useful for controlling the scanning of oblique record tracks across a magnetic tape in which video signals have been recorded.
More particularly, pilot signals of different frequencies, such as frequencies f.sub.1, f.sub.2, f.sub.3 and f.sub.4, all easily separable from recorded video signals, as by way of band pass filtering, are recorded in successive tracks. Thus, pilot signal f.sub.1 is recorded in track #1, pilot signal f.sub.2 is recorded in track #2, pilot signal f.sub.3 is recorded in track #3, pilot signal f.sub.4 is recorded in track #4, pilot signal f.sub.1 is recorded in track #5, pilot signal f.sub.2 is recorded in track #6, and so on. When, for example, the signals recorded in track #2 are reproduced, if the transducer which scans track #2 is subjected to a tracking error and, thus, is off-center, a cross-talk pilot signal component will be reproduced. Thus, in addition to pilot signal f.sub.2, which is played back from track #2, a cross-talk component of pilot signal f.sub.1 may be reproduced from track #1 with a magnitude related to the tracking error of the transducer. Alternatively, if the transducer is subjected to a tracking error in the opposite direction such that a portion of track #3 is scanned thereby, a cross-talk component of pilot signal f.sub.3 will be reproduced in addition to pilot signal f.sub.2. In this example, the magnitudes of cross-talk pilot signals f.sub.1 and f.sub.3 are sampled and compared to each other, with the result of this comparison being representative of the tracking error direction (that is, whether the transducer which is scanning track #2 also is picking up track #1 or track #3). The result of this comparison produces a tracking error control signal which, when applied to suitable adjustment devices, such as a bimorph support for the transducer or tape transport mechanisms, tends to return the transducer to its on-center position while scanning track #2.
The foregoing tracking control arrangement (as well as a similar tracking control technique which has been proposed heretofore and which will be described below) detects a tracking error at each scan, or pass, of a transducer along a track. This tracking error detection is fed back and used immediately as a servo control signal to correct the detected error. Thus, if a tracking error is detected during each transducer scan, that error is corrected promptly. In so-called continuous error correction, any tracking error which is detected at any portion of the transducer scanning path is corrected substantially immediately; and it often happens that several such corrections are carried out while an individual transducer scans a single track. Thus, the transducer may be moved in the left and right directions relative to that track, thereby imparting flicker, jitter and other disturbances to the video picture derived from the video signal reproduced from that track.
Another drawback of the aforementioned tracking control technique is attributed, in part, to small differences in size and operating characteristics among the several transducers normally used in a VTR to scan successive tracks. The rotary scanning mechanism of most commercially available VTR's includes a rotary drum having two or four record/playback heads mounted thereon. While such heads may satisfy manufacturing tolerances, as a practical matter the recording/reproducing characteristics of one head may differ somewhat from those of another. Furthermore, the mounting position of one head on the rotary drum may differ ever so slightly from the mounting position of another. Consequently, one record track may be slightly narrower than an adjacent track and, as a result, even when a reproducing head exhibits an on-center scanning path, it may pick up a cross-talk pilot frequency component from an adjacent track. Accordingly, this picked up component will be interpreted as a tracking error, resulting in an immediate attempt to "correct" the scanning path of the transducer. The resulting irregular scanning of the record tracks tends to repeat cyclically; for example, after the scanning of four successive tracks in which four different pilot signals are respectively recorded. Immediate attempts to "correct" what is perceived to be a tracking error results in zig-zag scanning of the record tracks, either by moving the bimorph transducer support or by changing the tape transport speed, resulting in wow and flutter which produces flicker and jitter in the displayed video picture.
It is believed that the foregoing drawbacks can be minimized, without reducing the efficacy of tracking control, by reducing the sensitivity of tracking control apparatus to instantaneous errors which may be detected as the transducer scans a record track.